Sodium ion solid-state conductors with sodium oxoferrate structure

ABSTRACT

A solid-state conductor with sodium oxoferrate structure is disclosed. The conductor may be used in battery applications where it is preferable to avoid the use of a liquid electrolyte. The conductor may be produced from an initial NaFeO2 chemical composition. So as to add defects and allow for sodium ion mobility, Fe(IV), Si, Sn, Ti, Zr, V, P, or S can be added. For example, (1−x)(NaFeO2)+x(XO2) can be melted with the corresponding oxide XO2, where X is Fe(IV), Si, Sn, Ti, Zr, V, P, or S, and x is between 0.1 and 0.5. These dopants generally preserve the crystallographic structure while decreasing the ion mobility barrier.

BACKGROUND

The present invention relates generally to the technical field ofsolid-state conductors, and, more specifically, relates to sodium ionsolid-state conductors.

As energy storage requirements become more demanding, next generationdevices will require a multitude of high performance battery products.Electrochemical energy storage is required for grid storage, wirelesscommunications, portable computing, and will be essential for therealization of future fleets of electric and hybrid electric vehicles.

Application areas, such as clean and renewable power, depend on newbattery technology for longer cycle life, higher energy densities,better recharge ability and increased reliability. In addition, therewill always be an environmental concern during production and useregarding safety and recycling. Further, since electrolytes in a batteryconduct ions, block electrons, and separate the electrodes to preventshorting, the electrolytes are an important part of a battery, and thedevelopment of high performance electrolytes will be significant forefficient battery technology, enhancement and broad applications.

Some batteries currently available use a liquid electrolyte containing aflammable organic solvent. As such, they require installation of asafety device to inhibit the temperature rise at the time of shortcircuit or improvement in technical structure or materials to inhibitshort circuit. In contrast, a battery having a solid material can avoidthis flammable solvent problem, and thereby simplify the safety deviceand reduce production cost and productivity.

Solid-state conductors that possess high ionic conductivity are neededfor a broad range of electronic and power applications. Applications nowalso may include chemical sensors, transistors, electromechanicalactuators, and light-emitting electrochemical cells. For someapplications, it is desirable to incorporate high ionic conductivitywhile maintaining certain mechanical properties. In looking at thosepossible materials that can be used for conductors in theseelectrochemical energy conversion and storage systems, variouscandidates have appeared. However, the state of the art considersmaterials that are of limited availability, are expensive, or whosechemical processing is not environmentally green.

For future applications, new solid-state materials with high ionic(lithium and sodium) conductivities are needed. Specifically, for sodiumion batteries there are only few materials available that are goodcandidates to replace the liquid electrolyte. Most recognize the majorclass of solid-state sodium ion conductors as NASICON. These materialsare based on Na—Zr—Si—P—O-based composite oxide, with the possibility ofdoping NASICON structures with Fe. However, these materials have thedrawback of being reactive with metallic sodium. Thus, there is need fora different class of materials.

SUMMARY

This disclosure is not limited to the particular systems, devices andmethods described, as these may vary. The terminology used in thedescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope.

In one or more embodiments, an apparatus comprises a solid-state ionconductor represented by the general formula NaFe_{¾}X_{¼}, where X isselected from the group consisting of Fe(IV), Si, Sn, Ti, Zr, V, P, andS. The solid-state ion conductor may comprise a thin film. The apparatusmay further comprise a first electrode in contact with the ionconductor, and a second electrode in contact with the ion conductor tocreate an electrochemical cell. Optionally, the apparatus may furthercomprise a first electrical lead in contact with the first electrode anda second electrical lead in contact with the second electrode. A firstcurrent collector in contact with the first electrode and a secondcurrent collector in contact with the second electrode may be added. Inan alternative embodiment, the apparatus may comprise a first electricallead in contact with the first current collector and a second electricallead in contact with the second current collector. The apparatus maycomprise a load attached to the first electrical lead and the secondelectrical lead. In an optional embodiment, the solid-state ionconductor may comprise a thin film, the first electrode may comprise athin film, and the second electrode may comprise a thin film.

In a preferred embodiment, the first electrode may comprise a layer ofnegative solid-state material adapted for electrochemically adsorbingand desorbing lithium ions during charge and discharge, and the secondelectrode may comprise a layer of positive solid-state material adaptedfor electrochemically desorbing and adsorbing lithium ions during chargeand discharge. The layer of negative solid-state material may comprise alayer of lithium metal, and the layer of positive solid-state materialmay comprise a layer of V₂O₅.

In other embodiments, an apparatus comprises a solid metal sodium unitand a coating on at least one surface of the solid metal sodium unitwherein the coating comprises a material represented by the generalformula NaFe_{¾}X_{¼}, wherein X is selected from the group consistingof Fe(IV), Si, Sn, Ti, Zr, V, P. and S. In a preferred embodiment, theapparatus may further comprise an electrical terminal attached to thesolid metal sodium unit, and the electrical terminal may penetrate thecoating, and the coating may surround the solid metal sodium unit. In anoptional embodiment, a wire can be attached to the solid metal sodiumunit, and the coating can surrounds the solid metal sodium unit and aportion of the wire. The coating may comprise a thin film.

In an alternative embodiment, a method for forming a solid-stateconductor comprises melting (1−x)(NaFeO2)+x(XO₂) with XO₂, where X isselected from the group consisting of Fe(IV) (iron), Si (silicon), Sn(tin), Ti (titanium), Zr (zirconium), V (vanadium), P (phosphorus), andS (silicon), and x is between 0.1 and 0.5. In a preferred embodiment, xis 0.25.

Numerous other embodiments are described throughout herein. All of theseembodiments are intended to be within the scope of the invention hereindisclosed. Although various embodiments are described herein, it is tobe understood that not necessarily all objects, advantages, features orconcepts need to be achieved in accordance with any particularembodiment. Thus, for example, those skilled in the art will recognizethat the invention may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taught orsuggested herein without necessarily achieving other objects oradvantages as may be taught or suggested herein.

The methods and systems disclosed herein may be implemented in any meansfor achieving various aspects, and may be executed in a form of amachine-readable medium embodying a set of instructions that, whenexecuted by a machine, cause the machine to perform any of theoperations disclosed herein. These and other features, aspects, andadvantages of the present invention will become readily apparent tothose skilled in the art and understood with reference to the followingdescription, appended claims, and accompanying figures, the inventionnot being limited to any particular disclosed embodiment(s).

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and the invention may admit toother equally effective embodiments.

FIG. 1 illustrates a crystallographic structure of an embodiment of thepresent invention.

FIG. 2 illustrates a schematic representation of a fully solid-statsodium battery, according to an embodiment of the present invention.

FIG. 3 illustrates a schematic representation of a metallic anodeprotected with a sodium-ion conducting material, according to anembodiment of the present invention.

FIG. 4 shows an energy profile along the diffusion pattern of Na atomsin the crystallographic materials, according to embodiments of thepresent invention.

Other features of the present embodiments will be apparent from theDetailed Description that follows.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a parthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.Electrical, mechanical, logical and structural changes may be made tothe embodiments without departing from the spirit and scope of thepresent teachings. The following detailed description is therefore notto be taken in a limiting sense, and the scope of the present disclosureis defined by the appended claims and their equivalents.

Various embodiments of solid-state conductors with sodium oxoferratestructures, electronic devices incorporating the solid-state conductors,and associated methods of manufacturing are described below. The presentinvention, in embodiments, is based on Na—Fe—O-based composite oxides,properly substituting part of Fe with Si, Sn, Ti, Zr, V, P, or S.

In embodiments, solid-state conductors with sodium oxoferrate structuresare used to refer to a solid material that is capable of transportingions and/or other charge carriers to effect ionic and/or other types ofconductivity, collectively referred to herein as “electricalconductivity.” A person skilled in the relevant art will also understandthat the technology may have additional embodiments, and that thetechnology may be practiced without several of the details of theembodiments described below.

By using a cognitive approach, two new materials are described that arethermodynamically stable with low ion diffusion barrier and lowelectronic conductivity to be used for sodium battery application. Thepresent invention, in embodiments, will help the creation of acompletely solid-state alternative to liquid-filled and polymer-gelbatteries. The technological advancement of the present invention mayassist in enabling performance enhancements through the reduction ofinert materials in the battery and improve safety through theelimination of flammable organic liquids.

The new materials for solid-state conductors are good ionic conductors,are thermodynamically stable, and have low electronic conductivity.These new materials involve modification of simple sodium oxoferrate,which is widely available, cheap, and easy to manufacture.

The initial material has chemical composition NaFeO₂. This material hasbeen screened through a database of crystallographic structures for useas a solid-state ionic conductor. NaFeO₂ has 3D channels populated bysodium ions with no defects and very low conductivity. In order tocreate defects and allow for sodium ion mobility, a new class ofmaterials: NaFe_{¾}X_{¼} with X=Fe(IV), Si, Sn, Ti, Zr, V, P, or S hasbeen developed. These materials are synthesized, in embodiments, byappropriately melting 3*(NaFeO₂) with the corresponding oxide (XO₂).However, various melts can be made according to the formula(1−x)(NaFeO₂)+x(XO₂), where X=Fe(IV), Si, Sn, Ti, Zr, V, P, or S and xis between 0.1 and 0.5, Formulations where x=0.25 are preferred incertain embodiments. The choice of the different “doping” elements hasbeen made in order to preserve the crystallographic structure and todecrease the ion mobility barrier.

FIG. 1 illustrates a crystallographic structure 100 of an embodiment ofthe present invention. The figure shows the Fe 110, O 120, Na 130, anddopant 140 atoms arranged in the crystal structure 100. The dopant, inembodiments, can be Fe(IV), Sn, Si, Ti, Zr, V, P, or S.

FIG. 2 illustrates a schematic representation of a fully solid-statesodium battery 200, according to an embodiment of the present invention.The electrochemical cell packaging 210 contains a cathode currentcollector 220, an anode current collector 230, a cathode 240, an anode260, and the solid-state electrolyte 250. The components 220, 240, 250,260 and 230 are in contact with each other in order to create a battery.The anode collector 230 permits the electron to flow during dischargefrom anode 260 to cathode 240 via the external circuit 270 and cathodecollector 220. The solid-state electrolyte 250 is positioned between theanode and cathode. The electrochemical cell generates a current, whichcan be used to power an external circuit 270, such as an electricalload.

As shown in FIG. 2, ions are extracted from the anode 260 and migratetoward the cathode 240 via the solid-state electrolyte 250. The ionspass through the solid-state electrolyte 250 and are inserted into thecathode 240. As a result, a current flows from the cathode 240 to theanode 260. During charging, a charger can provide a charging current tothe cathode 240. The charging current will cause the ions to beextracted from the cathode 240 and move toward the anode 260.

In embodiments, the electrochemical cell package 210 and its contentscan be constructed using thin-film techniques. A substrate of thepackage is first provided. To enable electrical power to be withdrawnfrom the battery, a current collector film can be deposited on thesubstrate, and then the cathode film is deposited upon the collectorfilm. The electrolyte film is then deposited in place so as to cover thecathode film. An anode is deposited upon the previously formed films soas to directly overlie a substantial portion of the electrolyte. Aprotective covering as part of the package can then be placed over thetop surface of the anode. Additional current collectors, wires, leads,or electrical terminals can be added to the package to connect to theexternal circuit or electrical load. These wires can penetrate thepackage to provide access.

In an embodiment, the anode 260 can include a carbonaceous material(e.g., graphite), tin (Sn), Zinc (Zn), lead (Pb), antimony (Sb), bismuth(Bi), silver (Ag), gold (Au), and/or other element electrodeposited onand alloy with lithium (Li), or combinations thereof. In anotherembodiment, the anode 260 can also include a binary, ternary, or higherorder mixtures of the elements that can be electrode posited on andalloy with lithium (Li). Non-limiting examples of binary mixturesinclude Sn—Zn, Sn—Au, Sn—Sb, Sn—Pb, Zn—Ag, Sb—Ag, Au—Sb, Sb—Zn, Zn—Bi,Zn—Au, and combinations thereof. Non-limiting examples of ternarymixtures include Sn—Zn—Sb, Sn—Zn—Bi, Sn—Zn—Ag, Sn—Sb—Bi, Sb—Zn—Ag,Sb—Zn—Au, Sb—Sn—Bi, and combinations thereof. A non-limiting example ofa quaternary mixture can include Sn—Zn—Sb—Bi. In yet another embodiment,the anode 260 can include intermetallic compounds of elements (e.g., thegenerally pure elements discussed above) and other elements that can beelectrodeposited and alloy with lithium (Li). Non-limiting examples ofsuch intermetallic compounds include Sn—Cu, Sn—Co, Sn—Fe, Sn—Ni, Sn—Mn,Sn—In, Sb—In, Sb—Co, Sb—Ni, Sb—Cu, Zn—Co, Zn—Cu, Zn—Ni, and combinationsthereof. The anode is adapted for electrochemically adsorbing anddesorbing lithium ions during charge and discharge.

The cathode 240 can be constructed from a layered oxide (e.g., lithiumcobalt oxide (LiCoO2)), a polyanion (e.g., lithium iron phosphate(LiFePO4)), or a spinel (e.g., lithium manganese oxide (LiMnZO4)). Othersuitable materials for forming the cathode 240 can include lithiumnickel oxide (LiNiOZ), lithium iron phosphate fluoride. The cathode isadapted for electrochemically desorbing and adsorbing lithium ionsduring charge and discharge.

In alternative embodiments, the fully solid-state sodium battery 200 canalso include insulators, gaskets, vent holes, and/or other suitablecomponents (not explicitly shown, but implied).

FIG. 3 illustrates a schematic representation of a metallic anode 300protected with a sodium-ion conducting material, according to anembodiment of the present invention. A metallic Na bar 310 is coveredwith a Na-ion conducting coating 320. The bar 310 may be solid metalsodium. A wire 330 or terminal composed of a conducting material canpenetrate or otherwise pass through or penetrate the Na-ion conductingcoating 320 to connect to the Na bar 310. The coating 320 can helpprotect the bar 310 from corrosion or other effects.

FIG. 4 shows an exemplary energy profile 400 along the diffusion patternof Na atoms in the crystallographic materials, according to embodimentsof the present invention. The x-axis is the diffusion pattern of Naatoms, and the y-axis is the energy profile. The different curvesrepresent the different elemental substitution according to theNaFe_{¾}X_{¼} with X=Fe(IV), Si, Sn, Ti, Zr, V, P, or S formulations.The barriers and with respect to the original structure were computerand barrier for mobility decreasing from 0.5 eV for the originalmaterial to approximately 0.25 eV for X=Si and X=Sn was reported. Boththese materials also show a higher band-gap with reduced electronmobility.

While the foregoing written description of the invention enables one ofordinary skill to make and use what is considered presently to be thebest mode thereof, those of ordinary skill will understand andappreciate the existence of alternatives, adaptations, variations,combinations, and equivalents of the specific embodiment, method, andexamples herein. Those skilled in the art will appreciate that thewithin disclosures are exemplary only and that various modifications maybe made within the scope of the present invention. In addition, while aparticular feature of the teachings may have been disclosed with respectto only one of several implementations, such feature may be combinedwith one or more other features of the other implementations as may bedesired and advantageous for any given or particular function.Furthermore, to the extent that the terms “including”, “includes”,“having”, “has”, “with”, or variants thereof are used in either thedetailed description and the claims, such terms are intended to beinclusive in a manner similar to the term “comprising.”

Other embodiments of the teachings will be apparent to those skilled inthe art from consideration of the specification and practice of theteachings disclosed herein. The invention should therefore not belimited by the described embodiment, method, and examples, but by allembodiments and methods within the scope and spirit of the invention.Accordingly, the present invention is not limited to the specificembodiments as illustrated herein, but is only limited by the followingclaims.

From the foregoing, it will be appreciated that specific embodiments ofthe disclosure have been described herein for purposes of illustration,but that various modifications may be made without deviating from thedisclosure. In addition, many of the elements of one embodiment may becombined with other embodiments in addition to or in lieu of theelements of the other embodiments. Accordingly, the technology is notlimited except as by the appended claims.

This disclosure is not limited to the particular systems, devices andmethods described, as these may vary. The terminology used in thedescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope.

In the above detailed description, reference is made to the accompanyingdrawings, which form a part hereof. In the drawings, similar symbolstypically identify similar components, unless context dictatesotherwise. The illustrative embodiments described in the detaileddescription, drawings, and claims are not meant to be limiting. Otherembodiments may be used, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presentedherein. It will be readily understood that the aspects of the presentdisclosure, as generally described herein, and illustrated in theFigures, can be arranged, substituted, combined, separated, and designedin a wide variety of different configurations, all of which areexplicitly contemplated herein.

The present disclosure is not to be limited in terms of the particularembodiments described in this application, which are intended asillustrations of various aspects. Many modifications and variations canbe made without departing from its spirit and scope, as will be apparentto those skilled in the art. Functionally equivalent methods andapparatuses within the scope of the disclosure, in addition to thoseenumerated herein, will be apparent to those skilled in the art from theforegoing descriptions. Such modifications and variations are intendedto fall within the scope of the appended claims. The present disclosureis to be limited only by the terms of the appended claims, along withthe full scope of equivalents to which such claims are entitled. It isto be understood that this disclosure is not limited to particularmethods, reagents, compounds, composition or logical systems, which can,of course, vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to be limiting.

As used in this document, the singular forms “a,” “an,” and “the”include plural references unless the context clearly dictates otherwise.Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art. Nothing in this disclosure is to be construed as anadmission that the embodiments described in this disclosure are notentitled to antedate such disclosure by virtue of prior invention. Asused in this document, the term “comprising” means “including, but notlimited to.”

While various compositions, methods, and devices are described in termsof “comprising” various components or steps (interpreted as meaning“including, but not limited to”), the compositions, methods, and devicescan also “consist essentially of” or “consist of” the various componentsand steps, and such terminology should be interpreted as definingessentially closed-member groups.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims bodies of theappended claims, are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes, but is not limited to”etc.). It will be further understood by those skilled in the art that ifa specific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should he interpreted to mean “at least one”or “one or more”); the same holds true for the use of definite articlesused to introduce claim recitations. In addition, even if a specificnumber of an introduced claim recitation is explicitly recited, thoseskilled in the art will recognize that such recitation should beinterpreted to mean at least the recited number (e.g., the barerecitation of “two recitations,” without other modifiers, means at leasttwo recitations, or two or more recitations). Furthermore, in thoseinstances where a convention analogous to “at least one of A, B, and C,etc.” is used, in general such a construction is intended in the senseone having skill in the art would understand the convention (e.g., “asystem having at least one of A, B, and C” would include but not helimited to systems that have A alone, B alone, C alone, A and Btogether, A and C together, B and C together, and/or A, B, and Ctogether, etc.). In those instances where a convention analogous to “atleast one of A, B, or C, etc.” is used, in general such as constructionis intended in the sense that one having skill in the art wouldunderstand the convention (e.g., “a system having at least one at A, B,or C” would include but not be limited to systems that have A alone Balone, C alone, A and B together, A and C together, B and C together,and/or A, B, and C together, etc.). It will be further understood bythose skilled in the art that virtually any disjunctive word and/orphrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.”

In addition, where features or aspects of disclosure are described interms of Markush groups, those skilled in the art will recognize thatthe disclosure is also thereby described in terms of any individualmember or subgroup of members of the Markush group.

As will be understood by those skilled in the art, for any and allpurposes, such as in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” and the like include the number recited andrefer to ranges which can be subsequently broken down into subranges asdiscussed above. Finally, as will be understood by one skilled in theart, a range includes each individual member. Thus, for example, a grouphaving 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, agroup having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells,and so forth.

Various of the above-disclosed and other features and functions, oralternatives thereof, may be combined into many other different systemsor applications. Various presently unforeseen or unanticipatedalternatives, modifications, variations or improvements therein may besubsequently made by those skilled in the art, each of which is alsointended to be encompassed by the disclosed embodiments.

1. An apparatus, comprising: a solid-state ion conductor represented bythe general formula NaFe_(3/4)X_(1/4), wherein X is selected from thegroup consisting of Fe(IV), Si, Sn, Ti, Zr, V, P, and S.
 2. Theapparatus of claim 1, wherein the solid-state ion conductor comprises athin film.
 3. The apparatus of claim 1, further comprising: a firstelectrode in contact with the ion conductor; and a second electrode incontact with the ion conductor.
 4. The apparatus of claim 3, furthercomprising a first electrical lead in contact with the first electrodeand a second electrical lead in contact with the second electrode. 5.The apparatus of claim 3, further comprising a first current collectorin contact with the first electrode and a second current collector incontact with the second electrode.
 6. The apparatus of claim 5, furthercomprising a first electrical lead in contact with the first currentcollector and a second electrical lead in contact with the secondcurrent collector.
 7. The apparatus of claim 6, further comprising aload attached to the first electrical lead and the second electricallead.
 8. The apparatus of claim 3, wherein the solid-state ion conductorcomprises a thin film, the first electrode comprises a thin film, andthe second electrode comprises a thin film.
 9. The apparatus of claim 3,wherein the first electrode comprises a layer of negative solid-statematerial adapted for electrochemically adsorbing and desorbing lithiumions during charge and discharge.
 10. The apparatus of claim 3, whereinthe second electrode comprises a layer of positive solid-state materialadapted for electrochemically desorbing and adsorbing lithium ionsduring charge and discharge.
 11. The apparatus of claim 10, wherein thelayer of negative solid-state material comprises a layer of lithiummetal.
 12. The apparatus of claim 10, wherein the layer of positivesolid-state material comprises a layer of V₂O₅.
 13. An apparatus,comprising: a solid metal sodium unit; and a coating on at least onesurface of the solid metal sodium unit, wherein the coating comprises amaterial represented by the general formula NaFe_(3/4)X_(1/4), wherein Xis selected from the group consisting of Fe(IV), Si, Sn, Ti, Zr, V, P,and S.
 14. The apparatus of claim 13, further comprising an electricalterminal attached to the solid metal sodium unit.
 15. The apparatus ofclaim 14, wherein the electrical terminal penetrates the coating. 16.The apparatus of claim 13, wherein the coating surrounds the solid metalsodium unit.
 17. The apparatus of claim 13, further comprising a wireattached to the solid metal sodium unit, and wherein the coatingsurrounds the solid metal sodium unit and a portion of the wire.
 18. Theapparatus of claim 13, wherein the coating comprises a thin film.
 19. Amethod for forming a solid-state conductor, the method comprisingmelting NaFeO₂ with a corresponding oxide XO₂ according to the generalformula (1−x)(NaFeO₂)+x(XO₂), wherein X is selected from the groupconsisting of Fe(IV), Si, Sn, Ti, Zr, V, P, and S, and x is between 0.1and 0.5, to thereby form the solid-state conductor having a sodiumoxoferrate structure of the general formula NaFe_(3/4)X_(1/4).
 20. Themethod of claim 19, wherein x=0.25.